The present invention generally relates to flow control apparatus for subterranean wells and, in a preferred embodiment thereof, more particularly relates to packer and safety valve apparatus used to shut-in a subterranean gas storage well.
Previously retrieved natural gas is often stored in underground cavern structures, such as salt caverns or abandoned mine shafts, for later use. To effect such storage a gas storage well is constructed by extending a casing string from a surface wellhead structure downwardly through the earth to the subterranean cavern, and then pumping the gas downwardly through the casing string into the cavern. To keep the pressurized gas delivered to the cavern from simply forcing its way up the casing string when the pumping pressure is terminated, it is necessary to "shut-in" the well until retrieval of some or all of the stored gas is needed.
To effect this well shut-in a normally closed safety valve is typically installed in the casing string and sealed therein using a packing structure. When it is desired to retrieve some of the pressurized gas stored in the cavern, the valve is simply opened to allow the pressurized gas to flow upwardly through the temporarily opened valve, into the wellhead structure through the casing string portion above the opened valve, and to a suitable receiving structure through an outlet pipe connected to the wellhead. To terminate upward flow of the stored gas, the safety valve is returned to its normally closed position.
Conventional safety valves, and their associated packer structures, used to shut-in gas storage wells in the manner generally described above are subject to several well known problems, limitations and disadvantages. For example, the opening of the casing-installed safety valve is typically effected using a vertical valve opening rod extending through the casing string, connected at its lower end to the movable valve closure element, and connected at its upper end to a pressure-actuated piston disposed in the wellhead structure and used to downwardly drive the rod to open the safety valve.
The use of this conventional piston/rod mechanical valve operating structure has two primary limitations. First, the movable valve element in the safety valve is typically spring-biased upwardly toward its normally closed position. When it is desired to permit this movable valve element to return to its normally closed position, its biasing spring must move not only the valve element upwardly, but the actuating rod member as well. Since the weight of the rod is directly proportional to its length, and the required size and force of the valve biasing spring are proportional to the weight of the rod, the maximum depth at which the safety valve may be installed in the casing string is often undesirably limited.
Additionally, the necessity of positioning the rod-driving piston structure renders this mechanical opening portion of the overall safety valve system susceptible to damage from vandalism and motor vehicles or other heavy equipment striking the wellhead structure.
Another limitation associated with safety valve systems of this general type is related to their related anchor and packer structures used to lock and seal them within the casing string to prevent stored gas from flowing upwardly through the casing string externally around the installed safety valve. This packer-related limitation comes in several varieties. For example, in one conventional type of shut-in application, the safety valve and its associated packer structure must be separately installed within the casing, using mechanical tools to set and later release the packer structure. In another type of valve and packer system, the packer portion of the shut-in system may be hydraulically set, but must be mechanically released using a suitable wireline tool.
In view of the foregoing it can be readily seen that it would be desirable to provide improved safety valve and packer apparatus, for use in shutting-in gas storage wells, which eliminates or at least substantially minimizes the above-mentioned problems, limitations and disadvantages associated with conventional valve and packer shut-in apparatus of the general type described. It is accordingly an object of the present invention to provide such improved safety valve and packer apparatus.